The problem with degradation and breakage of the carbon black pellets and the build-up of carbon black dust and fines on surfaces of carbon black handling apparatus has existed since the beginning of automated feeding and weighing of carbon black for the injection into a mixer. This problem has become even more critical in recent years due to the more sophisticated and unique rubber compounds. These newer compounds require softer, more fragile carbon black pellets which, due to their softer nature, will disperse more thoroughly into the rubber batch. As the sophistication of rubber compounding technology has increased, complete dispersion of the carbon black is more critical than ever before. The softer the pellets, the more complete the dispersion within a given time frame during the mixing process.
In rubber mixing, various ingredients are injected into a mixer to be compounded into the rubber. By its nature, carbon black will not mix well in its original form. As produced, carbon black is a very fine (micron size) powder. If injected into the mixer in that form, it will simply float on top of the rubber and will not mix well into the rubber batch. In order to overcome this problem, the carbon black, at the point of manufacture, is made into pellets. In the pellet form, the carbon black will then mix well into the rubber. First, the pellets themselves will mix, then the pellets will break down into powder and the powder will complete the total dispersion of the carbon black in the rubber batch. The making of the pellets is in of itself a science. These small pellets are made according to very close specifications. By their hardness, mass strength, elasticity, as well as other technical considerations, mixing performance can be determined.
These pellets are extremely fragile and easily broken. When broken, the pellets become powder (normally called "fines"). As in their original form, these powders will not disperse into the batch, but rather will "float" on top of the rubber. These pockets of powders become major flaws in the final product which most often causes the total product to be scrapped. Secondly, when high concentrations of fines are allowed to enter the mixer the time involved to mix the batch can become indeterminate, thereby extending the manufacturing time to unacceptable levels. Further, the mixing requires very specific time and temperature control, otherwise the rubber will "cure" inside the mixer due to the higher the temperatures reached during an extended mixing time. It therefore becomes highly significant to the success of the mixing operation that the pellets be handled in the most gentle manner possible, that any dust (fines) not be allowed to accumulate within the handling and feeding equipment where it may break away and be fed into the mixer.
When the pellets are broken, the resultant powders (fines) are highly prone to adhering to any surface with which they come into contact. This occurs primarily at any point within the system where material must be held for further process. The carbon black is metered closely by weight in conformance with a precise recipe, dependent upon the type of rubber compound being mixed. In the typical operation, a batch weight ranging from a few pounds to 500 or more may be required, with a tolerance of plus or minus 1% of total batch weight. The bulk densities of the carbon black pellets may vary from 20 to 45 pounds per cubic foot. These weights, once conveyed into the weigh hopper, with variables both in bulk density and total amount must be fed within a very specific cycle time, usually less than 90 seconds. Should this time cycle become unpredictable, all downstream operations are jeopardized. The mixing process is a closely timed, continuous operation, each step dependent upon the timely completion of all preceding steps in the operation.
The "fines", if allowed to accumulate within the weigh hopper creates significant problems. First, the build-up within the hopper will break away from surfaces, be fed to the mixer where it will not mix thoroughly, creating very high reject levels in the final product. Secondly, the build-up chokes off the hopper, thereby reducing the ability of the hopper to deliver accurate amounts within the required time. Further, extreme levels of build-up on hopper surfaces may create excessive maintenance shut-down time for cleaning and servicing the hopper.
As far as is known, there have been many attempts to use various types of hopper configurations. Some types of known attempts are: Extreme slope angles on hopper sides, ordinarily 70 degrees or more; hoppers which attempt to fluidize the material and thereby not require any direct contact with any hard surfaces. Further attempts to prohibit build-up and pellet breakage have included vibrators, inclines, stainless steel and polished surfaces, synthetic coatings and laminations. In addition to these mechanical attempts, there have been several methods of applying differing electrical charges to various parts of the equipment in order to control the ionic attraction of the material. These attempts have added to the problem rather than contributing to the solution.
This invention solves the problem of the carbon black pellets sticking to the handling equipment during the rubber manufacturing process by the use of a smooth flexible hopper surface which by a physical motion or flexing of the surface during the dump cycle of the hopper acts to expel any of the micron size particles which tend to adhere to the hopper sides. The movement of the surface is believed to accomplish the task due to several events. It provides an immediate release of the limiting friction. Secondly, the invention significantly reduces the kinetic friction between the smooth hopper surfaces and the carbon black particles being released from the hopper. The reduced kinetic friction reduces greatly the abrading of the pellets, and it minimizes the opportunity for ionic attraction and minimizes charge transfer during movement of the carbon black out of the hopper. Also, carbon black dust is hygroscopic. The hopper surface, as it flexes, minimizes the opportunity for the particles to adhere due to surface moisture within the dust particles.
The entire hopper assembly is constructed in such a way as to take geodesic advantage of the hopper wall construction. It will provide an internal surface which when loaded, will increase in tension and upon dumping, will return to its original shape in a quick "snapping" action. This action creates a surface least likely to provide the initial opportunity for the adhesion of dust to the hopper due to higher kinetic friction, moisture induced adhesion and ionic attraction of the static particles laying on the hopper. Prior methods of trying to solve the problem do not understand nor contemplate the use of a natural acting flexible surface nor do they use the other features of this invention. There may have been hoppers fabricated with inner fabric liners in the past. However, with these devices, as far as is known, releasing action is externally motivated by air injection or vibratory equipment. These devices do not intend the use of the initial flexure of the liner which results in stored kinetic energy to provide an imparted impetus to the carbon black to release from the surface, but rather intend only to vibrate the remaining dust particles from the hopper after it has emptied. The objects of the present invention are to release cleanly, and quickly any carbon black adhering to the hopper surfaces and to provide a physical assistance to the material to move from the hopper at the desired time, and within a very close cycle demand.
Another object of this invention is to minimize the opportunity for formation of lumps and chunks of dust to accumulate in the hopper. The design of the unit provides a type and grade of fabric which has a surface which provides an extremely smooth, non-porous surface. This surface, together with the actions of the hopper panels upon releasing insures that carbon black is not allowed to accumulate within the hopper.
By inhibiting the build-up on the hopper surfaces, the hopper is able to maintain its design feed rate so that a consistent and predictable mixing cycle time is provided. Another object is to improve the quality of the mixes reduce or and eliminate fines concentrations entering the mixer which in other hoppers comes from build-up breaking away from internal hopper surfaces. Another object is to minimize maintenance demands in service shut-downs due to inability to expel completely carbon black which has caked on the internal surfaces of the hopper.
Other objects of the invention will be apparent from the following detailed disclosure.